Why the Days Seem Shorter as We Get Older: The ‘mind time’ is a sequence of images; the rate at which changes in mental images are perceived decreases with age

Why the Days Seem Shorter as We Get Older. Adrian Bejan. European Review, Mar 18 2019. https://doi.org/10.1017/S1062798718000741

Abstract: Why does it feel that the time passes faster as we get older? What is the physical basis for the impression that some days are slower than others? Why do we tend to focus on the unusual (the surprise), not on the ever present? This article unveils the physics basis for these common observations. The reason is that the measurable ‘clock time’ is not the same as the time perceived by the human mind. The ‘mind time’ is a sequence of images, i.e. reflections of nature that are fed by stimuli from sensory organs. The rate at which changes in mental images are perceived decreases with age, because of several physical features that change with age: saccades frequency, body size, pathways degradation, etc. The misalignment between mental-image time and clock time serves to unite the voluminous observations of this phenomenon in the literature with the constructal law of evolution of flow architecture, as physics.

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Perceptions

Among the most common human perceptions is that time passes faster as an indivi-dual becomes older. The days become shorter, and so do the years. We all have storiesof this kind, from the long days of childhood and the never-ending class hours inelementary school, to days, months and years that now pass in a blur. The mostcommon sayings convey this impression: Timesflies; Where did the time go?; Lastyear was yesterday; Growing up took forever; A watched pot never boils; etc.More subtle, and worth questioning is the impression that some days appear to passmore slowly than others. The‘slower’days are full of productivity, events, and mem-ories of what happened. If you did not notice this difference between slow days and fastdays, then you should pay attention to it, because in this difference lies the explanationfor the lifelong puzzle sketched in the preceding paragraph. The hint is that productivedays happen when the body and mind are rested, after periods of regular sleep, when inthe morning you look in the mirror and you see a younger you, not a tired you.Athletes learn the hard way the correlation between good rest and the speed of thepassing time. Lack of rest makes you miss plays, unable to anticipate, unable to seethe ball before it arrives. While sleep walking, the game is over before you know it.
Young students learn the same physical truth while taking exams during afixedtime interval. The rested mind has more time to go through the problems, tofindmistakes, to go back to the beginning, and try again. Lack of sleep, due to crammingthe night before the exam, makes the time pass faster during the exam period.Cramming does not pay, but rest does, which is why the good coach rests the teambefore the big game.Here is why this is important to you, the reader. Today, many young peopleexperience time distortion because they spend too much time on social media. Thishas serious consequences, ranging from sleep deprivation to mood changes andmental disorder. This is why an understanding of the physics basis of how humansperceive the passing of time is essential.PhysicsTime represents perceived changes in stimuli (observed facts), such as visual images.1,2The human mind perceives reality (nature, physics) through images that occur as visualinputs reach the cortex. The mind senses‘time change’when the perceived image chan-ges. The time arrow in physics is the goal-oriented sequence of changes inflow config-uration, the direction dictated by the constructal law.1-11The present is different from thepast because the mental viewing has changed, not because somebody’s clock rings.The‘clock time’that unites all the liveflow systems, animate and inanimate, ismeasurable. The day–night period lasts 24 hours on allwatches, wall clocks and belltowers. Yet, physical time is not mind time. The time that you perceive is not thesame as the time perceived by another. Why? Because the young mind receives moreimages during one day than the same mind in old age. Said another way, if thelifespan is measured in terms of the number of images perceived during life, then thefrequency of mental images at young age is greater than in old age (Figure 1). Hereis why this should be:

Figure 1.The misalignment between perceived time and clock time during lifetime.

The sensory inputs that travel into the human body to become mental images–‘reflections’of reality in the human mind–are intermittent. They occur at certaintime intervals (t1), and must travel the body length scale (L) with a certain speed (V).In the case of vision,t1is the time interval between successive saccades. The timerequired by one mental image to travel from a sensory organ to the cortex is of ordert2~L/V. During life, the body length scale (L) increases in proportion with the bodymassMraised to the power 1/3, and, like all growth phenomena, the body massincreases over time in S-curve fashion,12monotonically, slow–fast–slow, cf.Figure 2.

Figure 2.All growth phenomena (spreading, collecting) exhibit an S-shaped historycurve8: fourflow systems where the size of theflow space increases monotonically,slow–fast–slow.


Figure 3.The length of theflow path increases as the body size and complexityincrease.9

The length traveled by inputs from external sensors to the cortex is actually greaterthanL, and it increases with age. The reason is that the complexity of theflow pathneeded by one signal to reach one point on the cortex increases as the brain grows andthe complexity of the tree-shapedflow paths increase, cf. Figure 3.13The broad trend then is thatLincreases with age. At the same time,Vdecreasesbecause of the ageing (degradation) of theflow paths. The key feature is that thephysical time (the combined effect oft1andt2) required by the occurrence of one mentalimage increases monotonically during the life of the individual. The frequency ofmental images decreases monotonically, and non-uniformly (i.e. not at constant rate).This trend is illustrated qualitatively in Figure 1. Two summarizing conclusions follow.(i)  More of the recorded mental images should be from youth.(ii)  The‘speed’of the time perceived by the human mind should increaseover life. The rate at which the physical time clock‘ticks’during onechange in the mental image increases with age.ReviewThe misalignment of the clock ticks and the changes perceived by the mind (Figure 1)brings together numerous observations and measurements accumulated in the lit-erature, especially in the study of vision and cognition.First, to define the terms, Fischer and Weber explain that during natural viewingconditions a normal adult subject makes 3–5 saccades in a second separated by per-iods of 200-300 ms during which the eyes do not make large or fast movements.14These periods are usually called‘fixations’. If the retinal image, as a whole, is pre-vented from moving (by successful voluntary attempts not to move the eyes, or bytechnical means), vision rapidly becomes blurred and the perception of the retinalimage fades away completely within 10 seconds. Fischer and Weber14explain that thehighly inhomogeneous structure of the primate retina, with an extremely high densityof receptor and ganglion cells in the center, a specialized fovea, and a rapid decline ofthe cell densities toward the periphery, makes it almost impossible to have a homogeneous and simultaneous percept of the total visualfield without somehowmoving the fovea to different positions and acquiring and integrating informationfrom these successive‘looks’. The existence of a fovea requires both eye movementsand periods offixation, that is, the active suppression of saccadic eye movements.Although the reaction times of saccades is relatively stable (200–250 ms),15theinfantfixation times are shorter than in adults. In primates, there is a constant rela-tionship between the duration, peak velocity and amplitude of saccadic eye move-ment,16–18known as the‘main sequence’, in which saccade trajectories have evolvedtoward optimizing the trade-off between accuracy and duration (speed) of the eyemovement. This is also in accord with the physics basis for the human preference fordisplays shaped in‘golden-ratio’rectangular frames, which is the shape that isscanned the fastest by the two human eyes.2As a result of an interaction betweenafferent, central and efferent neural processes we perceive a complete and stablevisualfield, which can serve as a frame within which we see motion and within whichwe move ourselves or parts of our body.14Bahill and Stark showed that fatigue can produce overlapping saccades in whichthe high-frequency saccadic bursts should show large pauses, glissades in which thehigh-frequency bursts should be much shorter than appropriate for the size of theintended saccades, and low-velocity, long-duration, non-Main Sequence saccades inwhich the mononeuronal bursts should be of lower frequency and longer durationthan normal.19When the saccadic eye movement system fatigues, saccades becomeslower, and the neurological control signal stratagem changes. The term fatigue isused in a broad sense, as it was by McFarland:‘a group of phenomena associatedwith impairment, or loss, of efficiency and skill’.20The intuitive view that the world is processed as a seamless stream of ongoingperception has been challenged in the current literature. Herzoget  al. discussedexperimental evidence supporting the view that perception might be discrete, furthersupporting evidence for discrete theories.21Visual information processing is similarto a sample and hold mechanism in engineering, as in analog/digital converters.Herzoget al. also noted that the brain functions such that we consciously perceiveonly the most plausible solution, and not a confusing manifold of possibilities thatoccur during unconscious processing.21The unconscious feature integration period isthe period of sense-making. The discrete conscious perception is followed byunconscious processing over time. These two modes of absorbing inputs from thesurroundings are analogous to all otherflows from point (e.g. eye) to volume (e.g.brain).1Observing fast and then letting it sink in slowly, is the same dynamicflowdesign as the long and fast, and short and slow that inhabits all nature,1animate andinanimate. Conscious perception and the unconscious processing that follows are the‘invasion’and‘consolidation’phases of the universal S-curve phenomenon.12Cicchiniet al. review the classical model of time perception, which considers asingle centralized clock that ticks at a constant rate.22They point out that muchexperimental evidence seems to cast doubt on this model. The ability to pay‘atten-tion’could modulate the tick rate and hence the duration of the events.23Manystudies found that the most surprising stimulus within a train of events is perceived longer, probably because it engages more transient attention or because the event isless predictable. Brunoet al. show that the apparent duration of moving visual objectsis greater at higher than at lower speeds.24Pöppelet al. argued that cognitive processes cannot be understood without theirtemporal dynamics; furthermore, certain logistical problems the brain has to deal withrequire an understanding of temporal processing.25Eaglemanet al. discussed theflash-lag illusion, where aflash and a moving object in the same location appear to beoffset.26They proposed an alternative in which visual awareness is neither predictivenor online but is postdictive, so that the percept attributed to the time of theflash is afunction of events that happen in the ~ 80 ms after theflash. Interpolation of the past isthe only framework that provides a unified explanation for theflash-lag phenomenon.VanRullen and Koch reconciled the unduly abandoned topic of discrete percep-tion with current views and advances in neuroscience.27Hainlineet al. showed thatfor both infants and adults, linear relationships were found between the peak velo-cities of fast eye movements and their amplitudes (main sequences).28With regard to the effect of aging, Sharpe and Zackon investigated horizontalsaccades in young, middle-aged and elderly normal subjects.29Saccades were elicitedin response to three target conditions: predictable amplitude direct and timing;unpredictable amplitudes and directions at regular intervals; and unpredictably timedtargets of predictable amplitude and direction. Peak velocities were significantlyreduced in the elderly when target amplitude and direction were predictable. Latencieswere prolonged in the elderly under all conditions. Saccadic accuracy was significantlydecreased in elderly subjects. Support for the thesis that age-related cognitive slowing isglobal is provided by Myersonet  al.30Although older adults perform worse thanyounger adults in complex decision-making scenarios, prior experience should be takeninto account in aging studies.31

Conclusion

Summing up, we conclude that the perceived misalignment between mental-imagetime and clock time (Figure 1) is in accord with and unifies the growing number ofobservations that describe aspects of this phenomenon in the literature. The physicsbasis is captured by the constructal law of evolution in nature (see Figures 2 and 3).

Acknowledgement
Professor Adrian Bejan’s research was supported by the US National ScienceFoundation.